1. Field of the Invention
The present invention relates to a direct positive silver halide photosensitive material, and in particular, to a direct positive silver halide photosensitive material which has a wide exposure latitude and a remarkably improved storage stability.
2. Description of the Prior Art
Methods are known wherein a reversed image can be obtained by chemically sensitizing the surfaces of internal latent image-type silver halide grains composed of (1) a silver halide core which is doped with a metal ion and/or chemically sensitized and (2) a silver halide shell coating at least a sensitive site of the core (hereinafter referred to as a core/shell-type grain), and then developing the core/shell-type grains in the presence of a fogging agent or according to a direct reversal method wherein the development is conducted under overall light exposure. Examples of such methods are described in U.S. Pat. Nos. 3,317,322 and 3,761,276.
The exposure latitude of direct positive photosensitive materials comprising the core/shell-type grains is widened usually by a process wherein emulsions of core/shell-type grains having various average grain sizes are prepared and mixed together, or by a process wherein a laminate of an outer layer comprising grains of a larger size and an inner layer comprising grains of a smaller size, is formed. The latter process is preferred to the former, since a wider exposure latitude can be obtained, and the amounts of stabilizer and spectral sensitizing dye adsorbable on the silver halide grains can be freely controlled in respect of the size of the grains to be used.
In applications of microphotography, duplication, etc., wherein good graininess and resolving power are required, silver halide grains having a smaller grain size are advantageous and the development of fine grain core/shell-type silver halide emulsion has been required. To produce such a fine grain core/shell-type emulsion, a process is known wherein the degree of chemical sensitization for the core is increased as described in Patent Disclosure No. 59-20854. However, such a wide exposure latitude photosensitive material using the smaller size core/shell-type silver halide grains in the fine grain core/shell-type emulsion has several defects. For example, when it is stored at a high temperature in an atmosphere of a high humidity or even at room temperature for a long period of time, its properties are largely changed. For example, its D.sub.max is reduced, the development speed is lowered, and a long development time is required for obtaining a sufficient D.sub.max.
For overcoming the above-mentioned defects, there has been proposed a process wherein a well-known stabilizer such as 4-hydroxy-6-methyl-1,3,3a,7-tetrazaindene or 1-phenyl-5-mercaptotetrazole is used, and processes which are directed to improving the storage stability of direct positive photosensitive materials, wherein a stabilizer such as those mentioned in Patent Disclosure Nos. 57-86829 and 60-95533, is used. However, these processes are still unsatisfactory, since sufficient effects cannot be obtained or D.sub.min is increased.